An adjuration consists of spoken words. But it is useless to speak to one that understands not, such as an irrational creature. Therefore it is vain and unlawful to adjure an irrational creature.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic understanding words
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“On the other hand, to repulse the demons by adjuring them, is to sever oneself from their fellowship. _______________________
THIRD ARTICLE [II-II, Q. 90, Art. 3]
Whether It Is Lawful to Adjure an Irrational Creature?
Objection 1: It would seem unlawful to adjure an irrational creature. An adjuration consists of spoken words. But it is useless to speak to one that understands not, such as an irrational creature. Therefore it is vain and unlawful to adjure an irrational creature. Obj. 2: Further, seemingly wherever adjuration is admissible, swearing is also admissible. But swearing is not consistent with an irrational creature. Therefore it would seem unlawful to employ adjuration towards one.
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